N-substituted maleimides are useful as starting materials or intermediates for the preparation of pharmaceuticals, agricultural chemicals, dyes, polymer compounds and the like. Recently, they have been used in large amounts to improve the heat resistance of styrene resins and are also widely used in copolymers for resin modification or polymer compounding.
There have been known a few methods for preparing N-substituted maleimides. A conventional preparation method comprises allowing maleic anhydride to react with an amine to obtain N-substituted maleamic acid which is then subjected to dehydrocyclization (imidization). As one example, there is a method in which N-substituted maleamic acid obtained by heating maleic anhydride and amine at 180° C. is subjected to dehydrocyclization [L. E. Coleman et al., J. Org, Chem., 24, 135˜136 (1959)]. However, this method is not economical, because the desired N-substituted maleimide is obtained in a yield of only about 15-50%.
Methods of preparing N-substituted maleimide in a relatively yield include methods in which a dehydrating agent or a dehydration catalyst is used for dehydrocyclization of N-substituted maleamic acid.
One example of the method that uses the dehydrating agent is disclosed in U.S. Pat. No. 2,444,536 in which N-substituted maleamic acid is dehydrocyclized using a dehydrating agent such as acetic anhydride in the presence of a sodium acetate catalyst. This method has a relatively high reaction yield, but it incurs a high production cost, because it uses a large amount of the dehydrating agent and carries out a complicated process of separating the product after the reaction. Thus, it is not an economical mass-production method.
Methods that are considered industrially advantageous include methods of dehydrocyclizing N-substituted maleamic acid under mild conditions using an effective dehydrating agent in place of the dehydrating agent. These methods do not use expensive sub-materials, and thus can be economically advantageous. U.S. Pat. No. 3,431,276 discloses a method of preparing N-substituted maleimides by heating and dehydrocyclizing N-substituted maleamic acid using an acid catalyst, such as sulfur trioxide, sulfuric acid or orthophosphoric acid, in a solvent having a suitable boiling point, without using a chemical dehydrating agent, and removing Thus produced water from the reaction system by azeotropic distillation. This method is advantageous in that an expensive dehydrating agent is not used and N-substituted maleimide can be easily separated, but the yield from the reaction is low and is likely to involve side reactions. In an attempt to improve this reaction yield, Japanese Patent Laid-Open Publication Nos. 53-68,770 and 57-42,043 disclose a method of preparing N-substituted maleimide by producing N-substituted maleamic acid, increasing the solubility of the N-substituted maleamic acid using an aprotic polar solvent such as dimethylformamide or dimethylsulfoxide and then dehydrocyclizing the N-substituted maleamic acid in the presence of an acid catalyst. The use of this aprotic polar solvent greatly increases the yield of N-substituted maleimide, but this solvent is expensive and highly toxic, and increases the production cost, because it is used in large amount. Also, this aprotic polar solvent can be deteriorated by the acid catalyst during the reaction, and thus needs to be used in a large amount. In addition, because this solvent has a high boiling point, much energy is required to separate the solvent from N-substituted male imide.
U.S. Pat. Nos. 4,623,734, 4,780,546 and 4,786,738 disclose methods comprising allowing maleic anhydride to react with amine in a non-polar organic solvent so as to produce N-substituted maleamic acid, and then dehydrocyclizing the N-substituted maleamic acid by azeotropic distillation in the presence of an acid stabilizer and other stabilizers. However, in the above methods, highly purified N-substituted maleimide cannot be prepared, and it is very difficult to separate the catalyst from the reaction solution after completion of the reaction due to the byproducts produced during the reaction. Even when the separation of the catalyst is achieved, the activity of the catalyst is rapidly reduced such that it cannot be effectively recycled. For the above reason, this method is not advantageous from an economical point of view.
U.S. Pat. No. 4,851,547 discloses supporting a catalyst on a solid carrier in order to facilitate the separation of the catalyst. This method appears to be suitable from an economical aspect, but reactors are required to be used separately, because maleic anhydride and amine are first allowed to react with each other to prepare N-substituted maleamic acid, and then an amine salt that was separately prepared is treated with acid and applied as a catalyst to the prepared N-substituted maleamic acid. Also, there is a problem that the N-substituted maleamic acid or the amine salt should be transferred in a slurry state. Furthermore, there is a problem in that a large amount of 2-amino-N-substituted succinimide is produced as a byproduct due to the use of an excessive amount of the amine during the reaction. In addition, because the amine that acts as a catalyst actually participates in the reaction, it is not suitable for the preparation of high-purity N-substituted maleimide.
U.S. Pat. No. 4,980,483 uses a two-stage reaction on the basis of the phenomenon in which the major byproduct 2-amino-N-substituted succinimide reacts with maleic anhydride so as to be selectively converted to N-substituted maleimide. According to the disclosure therein, in the former stage, a reaction is carried out using amine in an excess amount such that 2-amino-N-substituted succinimide can be produced, and in the latter stage, an excess amount of maleic anhydride is added, whereby N-substituted maleimide can be obtained in high yield and purity. However, a large amount of 2-amino-N-substituted succinimide is converted to N-substituted maleimide only under the condition that an excess amount of maleic anhydride is present, and non-converted 2-amino-N-substituted succinimide is an impurity present in the final product. For this reason, the above method is not suitable in a field requiring a high degree of purification. Also, 2-amino-N-substituted succinimide contained in the polymer compound can carbonize the surface of the polymer compound or make the surface of a final product irregular, and thus it should be removed.
U.S. Pat. No. 5,973,166 discloses a method in which a large amount of amine salt is used in place of an aprotic polar solvent to increase the solubility of reactants, intermediates and products in the prior-art maleimide preparation methods. However, this method has problems occurring when N-substituted maleamic acid is prepared first, like other preparation methods. Also, it has a relatively low yield and has a problem in that the purification process using recrystallization is complex. In addition, it is not advantageous from an economical viewpoint, because the expensive amine salt is used in large amount.